HOST PLANT RESISTANCE: HOW CAN WE MAKE BETTER USE OF IT IN A PHC PROGRAM

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1 HOST PLANT RESISTANCE: HOW CAN WE MAKE BETTER USE OF IT IN A PHC PROGRAM Fredric Miller, Ph.D. Professor of Horticulture Joliet Junior College And Senior Scientist Entomology The Morton Arboretum

2 What Do We Mean by Host Plant Resistance? Those characters that enable a plant to avoid, tolerate, or recover from attacks under conditions that would cause greater injury to other plants of the same species (Painter, 1951, 1958) Any plant trait that reduces the preference of herbivores or has a negative effect on the target herbivore (Strauss and Agarwal, 1999).

3 What Do We Mean by Tolerance Tolerance being the degree to which plant fitness is affected by herbivore damage relative to fitness in the undamaged state or the ability of the plant to regrow and/or reproduce after herbivory (Strauss and Agrawal, 1999).

4 Host Plant Resistance in the Real World Not a black and white phenomenon, but more of a spectrum of susceptibility and preference American elm is highly susceptible to Dutch elm disease, but new American elm cultivars and new Asian elm hybrids do not contract DED Princeton, Prairie Expedition, New Harmony, St. Croix, Valley Forge Accolade TM, Triumph TM, Danada Charm TM, Commendation TM, Cathedral

5 New American Elm Cultivars and Hybrid Elms

6 Host Plant Resistance in the Real World Certain native and non-native species of viburnum are preferred by viburnum leaf beetle Certain linden and crabapple taxa are preferred by Japanese beetle Green, black and white ash are highly susceptible to EAB, but blue ash appears to be resistant and Manchurian ash is rarely attacked

7 Why has HPR Been Slow to Be Implemented? Low demand from market place Focus has been on ornamental attributes High priority placed on plant beauty and looks

8 Why has HPR Been Slow to Be HPR requires a low aesthetic threshold Implemented? Great diversity of plant material andwide variety of pest and diseases Lack of research and funding

9 Direct Defenses Includes mechanical protection and production of toxic chemicals (secondary metabolites) Direct defenses are usually expressed as: Non-preference-an insect s response to host characteristics that lead away from the use of the host for food, oviposition, shelter Antibiosis-deleterious effects on insect survival or life history Tolerance-the ability of a host to grow and reproduce normally while supporting a pest population

10 Morphological and Mechanical Protection Waxy leaf cuticle Hairs and setae Trichomes Thorns

11 Morphological and Mechanical Protection Spines Lignification Leaf toughness Leaf thickness

12 Examples of Indirect Defenses Plant volatiles may be released below ground and protect plants from: Microbes Root-feeding insects Attract natural enemies Down-side: Exudates from trichomes may provide extra floral nectar (EFN) for squash bug

13 Chemical Warfare Primary Metabolites Essential for plant growth and function Occur in the major or primary metabolic pathways Consist of carbohydrates, lipids, proteins, and nucleic acids

14 Chemical Warfare Secondary Metabolites Not essential for plant growth, but by-products of metabolism Occur in the secondary metabolic pathways Derived from primary metabolites Consist of terpenoids, alkaloids, anthocyanins, phenols, quinones

15 Secondary Metabolites Inactive or stored as phytoanticipins Glucosinolates, benzoxazinoids, biocidal aglycones Activated as phytoalexins Isoflavonoids, terpenes, alkaloids Protect plants from stress, increase plant fitness, acts as deterrents, inhibit insect growth and development

16 TERPENES (HYDROCARBONS) Essential oils (i.e. herbs, perfumes, spices, incense) Resins (i.e. adhesives, varnishes, insecticides, rosin) Polyterpenes (i.e. latex, rubber)

17 ALKALOIDS

18 PHENOLICS (AROMATIC BENZENE RINGS) Flavonoids anthocyanins Tannins used for tanning leather Lignin gives cell walls their strength

19 GLYCOSIDES (GLUCOSE + NONSUGAR) Glucose + terpene Glucose + steroid Glucose + phenolic compound Saponins Shampoos and detergents Cardio active glycosides Digitoix and heart medicines Cyanogenic glycosides Contained in cassava Deadly poisons

20 Examples of Chemical Warfare Lignin (phenolic) limit pathogen entry by physically blocking or increasing leaf toughness Quinones (oxidized phenols) inhibit protein digestion and can be toxic Salicylic acid (SA) affects growth of winter moth larvae

21 Examples of Chemical Warfare Flavonoids help defend against abiotic and biotic stresses UV radiation, pathogens, insect pests Act as feeding deterrents, anti-feedants, possess antifungal properties Tannins bind to proteins, reduce nutrient absorption cause gut lesions in insects

22 Examples of Chemical Warfare Lectins (glycol-proteins) are toxic and interfere with digestion and nutrient absorption

23 Indirect Defenses Production and release of a mixture of volatile chemicals designed to: Attract parasitoids and predators of the pest insect Provide supplemental housing and food (extra floral nectar)

24 Examples of Indirect Defenses Activated by a combination of mechanical damage and elicitors from attacking insects Herbivore induced plant volatiles (HIPVs) include: Terpenes Green leafy volatiles (GLVs) Ethylene Methyl salicylates (Sas) GLVs and SAs attract predatory mites, big-eyed bug, ladybird beetles, and green lacewings

25 WHY DO INSECTS FEED ON SOME TREES AND BUT NOT OTHERS? WHAT ABOUT LEAF THICKNESS, TOUGHNESS, AND LEAF CHEMISTRY

26 Elm Leaf Beetle

27 Japanese Beetle, Gypsy Moth, Cankerworm, Elm Leafminer, Arborvitae Leafminer

28 WHAT HAVE WE LEARNED? There is a rich pool of Ulmus, Tilia, Quercus, Carpinus taxa for future tree breeding efforts Leaf morphology and chemistry appears to effect feeding preference and suitability and insect development Absence or presence of trichomes Leaf phenolic concentrations Leaf surface waxes Leaf toughness

29 LEAF THICKNESS AND TOUGHNESS FOR TILIA TAXA BY ORIGIN ORIGIN LEAF THCKNESS (mm.) INNER LEAF TOUGHNESS (kg) OUTER LEAF TOUGHNESS (kg.) ASIA 0.020a 0.025b 0.020a EUROPE 0.021a 0.019a 0.019a NORTH AMERICA 0.022a 0.022ab 0.020a Significance: NS F=8.1; P=0.02 NS

30 LEAF THICKNESS AND TOUGHNESS FOR ULMUS AND QUERCUS TAXA BY ORIGIN ORIGIN LEAF THICKNESS (mm.) INNER LEAF TOUGHNESS (kg) OUTER LEAF TOUGHNESS (kg.) ASIA 0.28b 0.032b 0.030b EUROPE 0.33b 0.025a 0.023a NORTH AMERICA 0.20a 0.025a 0.021a Significance: F=70.0; P<0.001 F-31.3; P<0.001 F=39.2; P<0.001 U. parvifolia EUR-NA OAKS b

31 What Have We Learned? Leaf toughness and thickness of Carpinus spp. was correlated with gypsy moth larval longevity and pupal weights

32 What Have We Learned? Elm leaves with greater chemical diversity were correlated with adult gypsy moth emergence and Japanese beetle feeding Gypsy moth emergence was correlated with leaf lipid diversity No significant correlation was found between elm leaf lipid diversity and Japanese feeding preference

33 What Have We Learned? Adult Japanese beetles frequently visited surfaces treated with a wax extract from preferred elm species compared to less preferred elm species

34 Host Evasion Host avoids a pest by passing through a susceptible stage before insect emergence or injury Utilizes pest biology and host plant phenology Example: elm leaf miner and elm phenology

35 Shape Plant Architecture and HPR Growth habit Height Canopy density Color

36 Reversing the Tables Bronze birch borer and white-bark birch Example of a native pest and a non-native plant North American birches had >70% survival Asian and European birches had 0% survival

37 Plant Stress and HPR Plants tend to release volatiles when under stress attracting: Bronze birch, honeylocust, and two-lined chestnut borers Conifer and hardwood bark beetles Outbreaks of bronze birch borer have been associated with drought

38 Plant Stress and HPR When under drought stress, EAB larvae performed better on Manchurian ash Conifers are vulnerable to bark beetle attacks when under stress due to reduced resin flow

39 What About Fertilization and HPR? (Herms, 2002) Common thought is fertilization enhances pest resistance Research data does not really support this practice Studies have shown fertilization can reduce plant resistance to pests and increase pest outbreaks Increases nutritional quality of host plant Reduces production of secondary metabolites

40 Growth-Differentiation Balance Hypothesis (GDBH) and HPR Postulates a physiological trade off between growth and secondary metabolism Predicts a parabolic response of secondary metabolism to variation in nutrient availability Fertilization of moderately nutrient-deficient plants may decrease secondary metabolism if growth is increased, but photosynthesis is not affected

41 Fertilization, Secondary Metabolites, and Photosynthesis Fertilization of extremely nutrient-limited plants may increase secondary metabolism if photosynthesis is also increased There is no strong evidence that fertilization increases tolerance to woody plant defoliation

42 Fertilization, Nitrogen, and Host Plant Resistance Rate of nitrogen (N) applied appears to be key factor affecting plant growth Form or method of application of N has shown little effect Suggests insect performance is influenced more by general plant response than fertilizer formulation

43 Prescription Fertilization and Host Plant Resistance Prescription fertilization can be highly effective and strongly recommended Fertilization programs must be tempered with knowledge and understanding of pest population dynamics and potential pest management consequences

44 Benefits of Using HPR Reduces use of chemical pesticides Reduces potential for pesticide resistance High economic value of ornamental plants

45 Benefits of Using HPR High cost of plant maintenance Potential sustainability and effectiveness Minimal impact on natural enemies Relatively low cost of implementation

46 Limitations of Using HPR Lengthy process involving years Strong desire for ornamental plant attributes High priority on aesthetics

47 Limitations of Using HPR Damage thresholds can be quite low Great diversity of ornamental plants and accompanying pests Lack of research and funding

48 Implementing HPR into a PHC Program Properly select and site plants Keep existing plants healthy Know your pest complex Use readily available plants suited for your area

49 Implementing HPR into a PHC Program Use native plants, where possible Select low maintenance plants that are not as susceptible to pests and diseases

50 THANK YOU FOR YOUR ATTENTION!